(1) Field of the invention
The present invention relates to a metal oxide magnetic substance, a magnetic film consisting of this magnetic substance, and their uses as magneto optical recording medium and perpendicular magnetic recording medium.
(2) Description of the prior art
In recent years, magneto optical recording mediums designed to carry out information recording by writing domains in a magnetic thin film making use of the thermal effects of light and read out information making use of magneto optical effects (Kerr effect, Faraday effect and the like) have been watched. Recording of information on this magneto optical recording medium is carried out making use of the rapid variation characteristic of a coercive force corresponding to the temperature variation in the neighborhood of the Curie temperature or compensation temperature of the magnetic substance. For instance, information is recorded as recording bit in the manner of radiating a laser beam modulated with a divalent signal selectively on a perpendicularly magnetized magnetic film under impression of magnetic field for heating the radiated portion over the Curie temperature, and thus reducing the coercive force on said portion for inverting the direction of magnetization. Regeneration, i.e. reading out, of the recorded information is carried out in the manner of radiating a polarized laser beam on the magnetic film, and thus detecting the direction of perpendicular magnetization from the difference in magneto optical effects between the recorded area and the non-recorded area, for instance the difference in Faraday's rotation angle.
The magnetic substance or magnetic film used in the magneto optical recording medium like this is required to have the following characteristics:
(1) Perpendicular magnetizability
(2) Powerful magneto optical effects
(3) Moderate coercive force permitting recording and regeneration by the laser beam and enhancing the memory stability (which is in the range of about 0.5-6 KOe. In case it is over 6 KOe, the impressing magnetic field required at the time of recording becomes powerful, while in case it is 0.3 KOe or less, the memory stability deteriorates.), and
(4) Moderate Curie temperature permitting recording and regeneration by the laser beam and enhancing the memory stability (which is in the range of about 100.degree.-400.degree. C. In case it is over 400.degree. C., recording by means of the laser beam becomes difficult, while in case it is less than 100.degree. C., the memory stability deteriorates.).
Usually, the magnetic substances used in magneto optical recording mediums were mostly occupied by those consisting of amorphous alloys made of rare earth metals and transition metals. The preparation of magneto optical recording mediums using such amorphous alloy magnetic substances was carried out generally in the manner of adhering said magnetic substance, for instance Tb-Fe alloy, on a substrate such as glass plate by vacuum vapor-deposition, sputtering or the like in the degree of about 0.1-1 .mu.m in thickness to form a magnetic film.
The magneto optical recording medium using aforesaid amorphous alloy magnetic substance is advantageous in that it can be recorded at high speed (at frequency=1 MHz) by means of a semi-conductor laser beam because it is of a high recording sensitivity, but is heavily disadvantageous in that the magneto optical characteristic of the magnetic film deteriorates with a lapse of time because the amorphous alloy magnetic substance, in particular the rare earth metal component, is subject to oxidizing corrosion. In order to prevent this, it is known to provide a protective layer consisting of SiO, SiO.sub.2 or the like on an amorphous magnetic film by means of vacuum vapordeposition, sputtering or the like, but this is disadvantageous in that at the time of preparing the magnetic film or the protective layer, the magnetic film is oxidized and corroded by O.sub.2 remaining in the vacuum, O.sub.2, H.sub.2 O or the like absorbed to the surface of the substrate, and O.sub.2, H.sub.2 O or the like contained in the target consisting of the alloy magnetic substance with a lapse of time, and further the oxidizing corrosion is enhanced by the light and heat at the time of recording. Further, the amorphous magnetic substance is disadvantageous in that it is liable to be crystalized by heat and therefore its magnetic characteristic is liable to be deteriorated. Still further, there has been brought about the problem that because this magnetic film is low in transmission coefficient in the laser-emitted wavelength region, the recorded information is read out by means of the magneto optical effects caused by reflection from the surface of the magnetic film, namely Kerr effect, but the obtained regeneration sensitivity is low because Kerr rotation angle is generally small.